Phthalic anhydride (PAN) is an important commercial chemical useful in the manufacture of plasticizers, polyesters, alkyd resins, and dyes. One important use is in the production of alkyl phthalates such as di-isononyl or di-isodecyl phthalates which are used as plasticizers typically for polyvinyl chloride. These phthalates may be further hydrogenated to the corresponding di-cyclohexanoates. Phthalic anhydride is typically produced from raw materials such as ortho-xylene (o-xylene) and naphthalene, suitable oxidation processes are disclosed in WO 2009/040245 and WO 2009/040246. The price of these raw materials and, as a direct result, the price of phthalic anhydride, has fluctuated greatly depending upon supply and demand. Because the cost of the raw materials is a major factor in the price of phthalic anhydride, it is of great importance that any system used to produce phthalic anhydride capture as much of the resultant product as possible. Phthalic anhydride may be successfully produced from any of a number of processes, i.e., (1) air oxidation of o-xylene in fixed-bed reactors, (2) air oxidation of petroleum or coal tar naphthalene in fixed-bed reactors, (3) fluid bed oxidation of o-xylene, (4) fluid bed oxidation of petroleum or coal tar naphthalene, and (5) liquid phase oxidation of o-xylene or naphthalene.
The general process scheme for the various vapor phase routes is to mix the hydrocarbon feed (in the vapor form) with compressed air and to feed the mixture to multi-tubular fixed-bed reactors which contain tubes packed with oxidation catalysts, e.g., vanadium oxide and titanium dioxide coated on an inert, nonporous carrier. When fluid bed reactors are used, the hydrocarbon feed in liquid form can be injected directly into the fluidized catalyst bed so that the air and the hydrocarbon are mixed in the reactor to produce a reactor effluent gas (i.e., the vapor phase oxidation product). The reactors are equipped with means for removing the heat of the oxidation reaction. The heat that is removed is used to generate steam.
After the vapor phase oxidation product exits either the fixed-bed or fluid bed reactors, it is cooled to cause the phthalic anhydride to condense. This allows separation of the phthalic anhydride from the gas stream.
In the production of phthalic anhydride, the reaction product exiting the multi-tubular fixed-bed or fluid-bed reactor containing the oxidation catalyst is a hot gas mixture containing among others nitrogen, water, CO2, and the desired phthalic anhydride. The reaction product is typically first cooled in a gas cooler, whereby most conveniently steam may be generated on the coolant side. The phthalic anhydride is usually recovered from the cooled reaction product by (de)sublimation in a switch condenser, a phase change that also may be called condensation or deposition, whereby the phthalic anhydride is collected as a solid on the switch condenser surface, usually the heat exchanger tubes, which are typically finned on the gas side to improve the heat transfer. The switch condenser is cooled with a cooling fluid, typically a thermal fluid or hot oil, capable of withstanding the high temperatures that are employed. After having been in collecting service, building up a layer of solid phthalic anhydride, typically on the outer surface of the finned exchanger tubes, the switch condenser may be switched from collecting service to melting service. Hereby, the gas flow through the switch condenser is usually discontinued and typically the cooling fluid is replaced by a heating fluid, usually the same thermal fluid or hot oil but now after heating, such that the phthalic anhydride melts and forms a liquid, and the liquid phthalic anhydride is drained and collected for further processing. The emptied switch condenser is cooled before putting it back into collecting service by replacing the heating fluid by the cooling fluid.
The phthalic anhydride is typically condensed as a solid. However, a two-stage condensation system may be used to first condense a portion of the phthalic anhydride as a liquid and then to condense the remainder as a solid. The process further comprises recovering phthalic anhydride from the reaction product mixture by a precondenser condensing phthalic anhydride as a liquid followed by a switch condenser collecting phthalic anhydride as a solid. The addition of a precondenser provides the advantage that the gaseous mixture is brought outside the explosive limits by reducing the concentration of the explosive components and by lowering the operating temperature to below the minimum ignition temperature for the resulting gaseous mixture, and this before the gas mixture enters the switch condensers. The precondenser preferably also contains finned tubes, and may be cooled with any type of cooling medium, for example, hot water may be used as it allows avoiding the occurrence of spots having too low temperatures and having the ability to control the precondenser outlet temperature within a narrow range. The outlet of the precondenser may be kept at a temperature of at least 137° C.
Machinery and equipment that operate under severe and varying conditions are often susceptible to fatigue, especially at areas that are the focal points of stress. Fatigue can result in a premature failure, requiring either repair or replacement, for example, the failure of components through the application of cyclical stress, in particular at the junctions of two or more components. Switch condensers are commonly used in the recovery of phthalic anhydride from a reaction gas. (See, for example, U.S. Pat. Nos. 4,435,580, 5,214,157, 5,508,443, 5,869,700, and Ser. No. 61/304,063, filed Feb. 12, 2010, to De Munck et al.). They operate in a cyclical service that includes a temperature gradient of from 50° C. to 200° C. or alternatively, from 60° C. to 180° C. They are susceptible to fatigue, especially, at junctions, due to the temperature variations caused by the condensing/cooling mode and the heating/melting mode extending a broad and varying temperature gradient through-out the body of the condenser.
To achieve the temperature cycles, the switch condensers comprise heavy tubular bundles that circulate fluids in order to accomplish the heat exchange. The internal heat exchanger bundles are supported by support members, generally, rectangular beams, connecting and extending through the outer walls of the shell of the switch condenser. Junctions are formed, where the support members cross the shell wall, these are typically made by welding. Cracking or failure of the junctions may occur due to stress caused by the temperature gradient in the switch condenser at the coupling of the support members and shell wall, for example, at the upper and lower beams with the shell wall. Severe failures such as through wall cracks cause leakage of phthalic anhydride and damage the shell by acid corrosion.
Background references include DE 10 72 965, DE 195 12 845, DE 195 46 702, EP 0 045 392 A, and GB 1 212 088.
Thus, there exists a need for a design modification that significantly reduces the magnitude of the stress at the junctions between the support members and the shell wall that eliminates or significantly reduces failures such as cracking at these locations.